1. Field of the Invention
This invention relates generally to a nozzle for an extreme ultraviolet (EUV) lithography source and, more particularly, to a nozzle for an EUV source that employs a target delivery tube within the nozzle to thermally isolate the target material from the heat generated by the plasma.
2. Discussion of the Related Art
Microelectronic integrated circuits are typically patterned on a substrate by a photolithography process that is well known to those skilled in the art, where the circuit elements are defined by a light beam propagating through a mask. As the state of the art of the photolithography process and integrated circuit architecture becomes more developed, the circuit elements become smaller and more closely spaced together. As the circuit elements become smaller, it is necessary to employ photolithography light sources that generate light beams having shorter wavelengths and higher frequencies. In other words, the resolution of the photolithography process increases as the wavelength of the light source decreases to allow smaller integrated circuit elements to be defined. The current state of the art for photolithography light sources generate light in the extreme ultraviolet (EUV) or soft X-ray wavelengths (13.4 nm).
Different devices are known in the art to generate EUV radiation. One of the most popular EUV radiation sources is a laser-plasma, gas condensation source that uses a gas, typically Xenon, as a laser plasma target material. Other gases, such as Krypton, and combinations of gases, are also known for the laser target material. The gas is forced through a nozzle, and as the gas expands, it condenses and converts to a liquid spray. The liquid spray is illuminated by a high-power laser beam, typically from an Nd:YAG laser, that heats the liquid droplets to produce a high temperature plasma which radiates the EUV radiation. U.S. Pat. No. 5,577,092 issued to Kubiak discloses an EUV radiation source of this type.
FIG. 1 is a plan view of a known EUV radiation source 10 including a nozzle 12 and a laser beam source 14. A gas 16 flows through a neck portion 18 of the nozzle 12 from a gas source (not shown). The gas 16 is accelerated through a narrowed throat portion and is expelled through an exit collimator of the nozzle 12 as a jet spray 26 of liquid droplets. A laser beam 30 from the source 14 is focused by focusing optics 32 on the liquid droplets. The energy of the laser beam 30 generates a plasma 34 that radiates EUV radiation 36. The nozzle 12 is designed so that it will stand up to the heat and rigors of the plasma generation process. The EUV radiation 36 is collected by collector optics 38 and is directed to the circuit (not shown) being patterned. The collector optics 38 can have any suitable shape for the purposes of collecting and directing the radiation 36. In this design, the laser beam 30 propagates through an opening 40 in the collector optics 38.
It has been shown to be difficult to produce a spray having large enough droplets of liquid to achieve the desired efficiency of conversion of the laser radiation to the EUV radiation. Because the liquid droplets have too small a diameter, and thus not enough mass, the laser beam 30 causes some of the droplets to break-up before they are heated to a sufficient temperature to generate the EUV radiation 36. Maximum diameters of droplets generated by a gas condensation EUV source is on the order of 0.33 microns. However, droplet sizes of about 1 micron in diameter would be desirable for generating the EUV radiation. Additionally, the large degree of expansion required to maximize the condensation process produces a diffuse jet of liquid, and is inconsistent with the optical requirement of a small plasma size.
To overcome the problem of having sufficiently large enough liquid droplets as the plasma target, U.S. Pat. No. 6,324,256, issued Nov. 27, 2001, titled xe2x80x9cLiquid Sprays as the Target for a Laser-Plasma Extreme Ultraviolet Light Source,xe2x80x9d discloses a laser-plasma, extreme ultraviolet light source for a photolithography system that employs a liquid spray as a target material for generating the laser plasma. In this design, the EUV source forces a liquid, preferably Xenon, through the nozzle, instead of forcing a gas through the nozzle. The geometry of the nozzle and the pressure of the liquid propagating through the nozzle, atomize the liquid to form a dense spray of liquid droplets. Because the droplets are formed from a liquid, they are larger in size, and are more conducive to generating the EUV radiation.
Another problem exists in the known EUV sources that causes some of the liquid target material to vaporize prior to being energized by the laser. The plasma generation area is typically about 2 mm away from the nozzle exit, and is generating heat at about 200,000xc2x0 K. Because the EUV radiation source nozzle is positioned so close to the plasma generation area, the heat from the plasma heats the nozzle and thus the target material therein. The nozzles are typically subjected to thermal inputs up to 10 kW/cm2. Warming the target material at the expansion aperture of the nozzle leads to reduced target production and to the formation of EUV absorbing vapors. Particularly, heating of the nozzle to such high temperatures causes some of the liquid target material to vaporize reducing the liquid density of the target. Further, particles from the plasma generation process cause a sputtering effect on the nozzle which adversely affects the EUV generation. It is known in the art to make the nozzle out of graphite to reduce the sputtering effects, although other materials may be used for better erosion resistance. However, graphite is a good thermal conductor which enhances heating of the cold target material within the nozzle.
What is needed is a nozzle for a laser-plasma EUV radiation source that provides thermal isolation between the nozzle body and the target material traveling therethrough to enhance the EUV radiation generation. It is therefore an object of the present invention to provide such an EUV radiation source nozzle.
In accordance with the teachings of the present invention, a nozzle for a laser-plasma EUV radiation source is disclosed that provides thermal isolation between the nozzle body and the target material flowing therethrough. A separate target material delivery tube protrudes through the nozzle body with limited tube/nozzle surface contact such that proper tube/nozzle alignment is achieved while providing thermal isolation. In one embodiment, the delivery tube is made of a material having low thermal conductivity, such as stainless steel, so that heating of the nozzle body from the plasma does not heat the liquid target material being delivered through the delivery tube. The delivery tube has an expansion aperture positioned behind an exit collimator of the nozzle body. The expansion aperture has a smaller diameter than the known exit collimators to deliver less material to the plasma generation area.
Additional objects, advantages and features of the present invention will become apparent to those skilled in the art from the following discussion and the accompanying drawings and claims.